1. Field of the Invention:
The present invention relates to an optical scanner and more particularly to improvements of a galvanometer type optical scanner.
2. Description of the Prior Art:
Optical scanners for effecting the deflective scanning of a laser beam or other optical beams have been currently used to object recognizing systems for reading two- or three-dimensional objects, image display systems, laser printers and other systems.
Such optical scanners are realized in various forms such as electrooptical type, acoustooptical type, hologram type or polyhedral rotary mirror type. Galvanometer type optical scanners also are known to perform the scanning at high speeds.
Such galvanometer type optical scanners are disclosed in Japanese Patent Publication No. 50-1402, Japanese Laid-Open Patent Application No. 51-87708 and Japanese Laid-Open Patent Application No. 53-143910 wherein a rotor is fixedly mounted on a torsion bar and includes a reflecting mirror provided therein. The rotor is electromagnetically driven to receive a beam of light on said mirror rotated therewith. In this way, the rotated mirror can provide a desired scanning light during the rotation of the rotor.
The galvanometer type optical scanners can perform the optical scanning at high speeds since the natural frequency of the oscillating system including the torsion bar is high. For example, in the recent laser printers, the galvanometer type optical scanner is very useful for providing a scanning beam of higher resolving power used to write a latent image on the surface of a photosensitive drum.
However, the prior art galvanometer type optical scanners is disadvantageous in that it must have a large-sized optical scanning system since the torsion bar having its length sufficient to reciprocate the rotor with the reflecting mirror therealong must be arranged opposed to a source of light and since the scanning beam must be directed to any desired section to be irradiated, for example, to the surface of a photosensitive drum in a laser printer. This provides a serious problem when the optical scanner is to be incorporated into various types of industrial machines.
Particularly, the prior art utilized an upright type optical scanning unit wherein said torsion bar was vertically arranged. In such an arrangement, it is difficult to reduce the size of the entire machine. It is also impossible to provide a torsion bar which is firmly fixed to the base of the machine. The latter leads to that a steady scanning beam cannot be provided due to vibration of the torsion bar.
In the aforementioned optical scanners, further, there is provided a yoke for producing a driving magnetic flux around the rotor to provide an electromagnetic drive force for the rotor. The static neutral position of the rotor is established at a position wherein the rotor is positioned opposed to the yoke. In such an arrangement, the electromagnetic drive force will be applied to the rotor when the latter is in its maximum speed position wherein the free vibration of the rotor is most stabilized, that is, the static neutral position. The free vibration of the rotor is thus adversely affected by the electromagnetic drive force to prevent the uniformity of velocity or the stability of free vibration from being provided. As a result, the speed of the scanning beam will become unstable.
The second problem in the prior art is that the rotor is attracted by the yoke since the electromagnetic drive force is applied to the rotor at its static neutral position. More particularly, the rotor is suspended from the torsion bar. Only the rotational motion of the rotor is thus required to perform the optical scanning operation. Since the rotor is suspended from the relatively long torsion bar, therefore, the shaft itself of the rotor tends to move easily. As a result, the electromagnetic drive force tends to move the rotor toward the pole end of the yoke rather than the rotation of the rotor. In the worst case, the rotor would be engaged by the yoke to stop the rotational motion of the rotor.
Even though the condition does not reach the above worst condition, the shaft of the rotor will be vibrated to provide an unstable scanning beam when the electromagnetic drive force is applied to the rotor at a position wherein the rotor is moved closer to the yoke.